Radioimmunoassays to the major structural protein and major glycoprotein of the endogenous guinea pig retrovirus (GPV) were used to show that GPV expression in guinea pig cells and tissues is not correlated to neoplastic transformation. Previously described interspecies immunoassays indicating that distantly related retroviral proteins contain common antigenic determinants has prompted the development of a search for monoclonal antibodies recognizing these determinants.